1. Technical Field
The present disclosure relates to diaphragms and loudspeakers and, particularly, to a diaphragm based on carbon nanotube and a loudspeaker using the same.
2. Description of Related Art
A loudspeaker is an acoustic device transforming received electric signals into sounds. There are different types of loudspeakers that can be categorized accordingly to their working principle, such as electro-dynamic loudspeakers, electromagnetic loudspeakers, electrostatic loudspeakers and piezoelectric loudspeakers. Among the various types, the electro-dynamic loudspeakers have simple structures, good sound qualities, low costs, and thus, are most widely used.
The electro-dynamic loudspeaker typically includes a diaphragm, a bobbin, a voice coil, a damper, a magnet, and a frame. The voice coil is an electrical conductor, and is placed in the magnetic field of the magnet. By applying an electrical current to the voice coil, a mechanical vibration of the diaphragm is produced by the interaction between the electromagnetic field produced by the voice coil and the magnetic field of the magnets, thus producing sound waves by kinetically pushing the air. The diaphragm reproduces the sound pressure waves, corresponding to the original input electric signals.
To evaluate the loudspeaker, sound volume is a decisive factor. The sound volume of the loudspeaker relates to the input power of the electric signals and the conversion efficiency of the energy. However, when the input power is increased to certain levels, the diaphragm could deform or even break, thereby causing audible distortion. Therefore, the strength and Young's modulus of the diaphragm are determining factors of a rated power of the loudspeaker. The rated power is the highest input power by which the loudspeaker can produce sound without audible distortion. Additionally, the lighter the weight per unit area of the diaphragm, the smaller the energy required for causing the diaphragm to vibrate, the higher the energy conversion efficiency of the loudspeaker, and the higher the sound volume produced by the same input power.
Accordingly, the higher the strength and the Young's modulus, and the smaller the density of the diaphragm, the higher the efficiency and volume of the loudspeaker.
However, the material of the diaphragm is usually polymer, metal, ceramic, or paper. The polymer and the paper have relatively low strength and Young's modulus. The metal and ceramic have relatively high weight. Therefore, the rated power of the conventional loudspeakers is relatively low. In general, the rated power of a small sized loudspeaker is only 0.3 W to 0.5 W. In another aspect, the density of the conventional diaphragms is usually large, thereby restricting the the energy conversion efficiency. Therefore, to increase the rated power and the energy conversion efficiency of the loudspeaker and to increase the sound volume, the improvement of the loudspeaker is focused on increasing the strength and Young's modulus and decreasing the density of the diaphragm. That is, to increase the specific strength (i.e., strength/density) and the specific Young's modulus (i.e., Young's modulus/density) of the diaphragm.
Carbon nanotubes (CNT) are a novel carbonaceous material having extremely small size, light weight, and extremely large specific surface area. Carbon nanotubes have received a great deal of interest since the early 1990s and have been widely used in a plurality of fields, because of their interesting and potentially useful electrical and mechanical properties. PCT patent application WO2007043837 published on Apr. 19, 2007 discloses a diaphragm of a loudspeaker using carbon nanotubes dispersed in a matrix material by the addition of surfactant, stearic acid or fatty acid, to improve the strength of the diaphragm. However, the carbon nanotubes are in a powder form. Due to the large specific surface area of the carbon nanotube, the carbon nanotube powder aggregates easily in the matrix material. Thus, the larger the ratio of the carbon nanotubes in the matrix material, the more difficult it is to disperse the carbon nanotubes. Further, the addition of the surfactant, stearic acid or fatty acid introduces impurities into the diaphragm. The dispersion of the carbon nanotube relates to complicated reaction processes.
What is needed, therefore, is to provide a diaphragm and a loudspeaker using the same with high strength and Young's modulus.